In connection with engine braking of a vehicle, the throttle and fuel supply to the combustion engine are shut off. When the air in the cylinders is compressed during the compression stroke, the pistons will, via the rods, exert a braking torque on the crankshaft, which during the engine brake process is operated by the vehicle's driving wheels via driving shafts, a propeller shaft and the transmission. Since the crankshaft is directly connected with the vehicle's driving wheels during the engine-braking process, the braking torque from pistons and rods, affecting the crankshaft, will therefore brake the vehicle during engine braking.
In order to reinforce the effect of the engine brake, the exhaust valves may be deactivated, so that they remain closed during the exhaust stroke. The air in the cylinders will thus be compressed also during the exhaust stroke, entailing that the braking torque from the pistons and rods, affecting the crankshaft, also arises during the exhaust stroke.
In order to utilize the braking energy in connection with engine braking, the pressure of the air compressed in the cylinders must be reduced at the end of each compression. This is carried out with a decompression device that controls the exhaust valves, so that they are opened at the end of the compression stroke and at the end of the expansion stroke. Therefore, the air compressed in the cylinders will leave the cylinder via the exhaust channels and further along through the exhaust system. The decompression device subsequently closes the exhaust valves, so that air may be sucked in through the inlet valves and an overpressure may be built up in the cylinders during the next compression.
When the exhaust valves are deactivated during the exhaust stroke, a very high pressure arises in the cylinders. When the subsequent inlet stroke is initiated, it is important that the high pressure in the cylinders has been reduced with the help of the decompression device before the inlet valves are opened. In the event the pressure in the cylinders exceeds a certain level as the inlet valves are opened, the inlet valves and the drivetrain connected to the inlet valves may fail, because of the substantial force which the inlet valves and its drivetrain must overcome in order to open the inlet valves in the cylinder.
In a combustion engine comprising several cylinders, it is possible to control the braking torque during engine braking, by controlling the deactivation of the exhaust valves and controlling the decompression device for each cylinder. For example, by deactivating the exhaust valves and activating the decompression device in half of the engine's cylinders, the braking torque will be halved. It is also possible to deactivate the exhaust valves in any number of the engine's cylinders. Control may thus be carried out in steps, wherein the number of controllable steps depends on the number of cylinders in the engine.
Under certain operating conditions in the vehicle, it would be desirable to carry out the control of the engine braking torque steplessly, in order thus to be able to engine-brake the vehicle comfortably.
Document WO 2004059131 shows a system for engine braking in a combustion engine, wherein an exhaust valve is opened on several occasions during engine braking.
Document WO 2012038195 pertains to an engine-braking system for a combustion engine, wherein the opening and closing of the exhaust valves is brought forward in time, following which an opening of the exhaust valves takes place after the closing, with the objective of increasing the engine brake effect.
Document U.S. Pat. No. 6,394,067 shows a combustion engine with double camshafts, wherein the opening of the exhaust valve is brought forward in time during engine braking. The exhaust valve is subsequently only partly closed, in order to be fully closed before it is opened to reduce the pressure in the cylinders.
Document U.S. Pat. No. 3,234,923 describes a method and an engine braking system for a combustion engine. A phase shift of a camshaft to control exhaust valves results in engine braking. The phase shift is approximately 160 degrees on the crankshaft, which entails that the exhaust valves are opened at a crankshaft position, corresponding to the exhaust valve opening achieved by the decompression device, as discussed above.